Cholesterol, lipid rafts, and disease

Abstract

Lipid rafts are dynamic assemblies of proteins and lipids that float freely within the liquid-disordered bilayer of cellular membranes but can also cluster to form larger, ordered platforms.Rafts are receiving increasing attention as devices that regulate membrane function in eukaryotic cells.In this Perspective, we briefly summarize the structure and regulation of lipid rafts before turning to their evident medical importance.Here, we will give some examples of how rafts contribute to our understanding of the pathogenesis of different diseases.For more information on rafts, the interested reader is referred to recent reviews (1, 2). Composition of lipid raftsLipid rafts have changed our view of membrane organization.Rafts are small platforms, composed of sphingolipids and cholesterol in the outer exoplasmic leaflet, connected to phospholipids and cholesterol in the inner cytoplasmic leaflet of the lipid bilayer.These assemblies are fluid but more ordered and tightly packed than the surrounding bilayer.The difference in packing is due to the saturation of the hydrocarbon chains in raft sphingolipids and phospholipids as compared with the unsaturated state of fatty acids of phospholipids in the liquid-disordered phase (3).Thus, the presence of liquid-ordered microdomains in cells transforms the classical membrane fluid mosaic model of Singer and Nicholson into a more complex system, where proteins and lipid rafts diffuse laterally within a two-dimensional liquid.Membrane proteins are assigned to three categories: those that are mainly found in the rafts, those that are present in the liquid-disordered phase, and those that represent an intermediate state, moving in and out of rafts.Constitutive raft residents include glycophos-phatidylinositol-anchored (GPI-anchored) proteins; doubly acylated proteins, such as tyrosine kinases of the Src family, Gα subunits of heterotrimeric G proteins, and endothelial nitric oxide synthase (eNOS); cholesterol-linked and palmitate-anchored proteins like Hedgehog (see Jeong and McMahon, this Perspective series, ref. 4); and transmembrane proteins, particularly palmitoylated proteins such as influenza virus hemagglutinin and β-secretase (BACE) (1).Some membrane proteins are regulated raft residents and have a weak affinity for rafts in the unliganded state.After binding to a ligand, they undergo a conformational change and/or become oligomerized.When proteins oligomerize, they increase their raft affinity (5).A peripheral membrane protein, such as a nonreceptor tyrosine kinase, can be reversibly palmitoylated and can lose its raft association after depalmitoylation (6).By these means, the partitioning of proteins in and out of rafts can be tightly regulated. Cholesterol and raft biogenesisCholesterol is thought to serve as a spacer between the hydrocarbon chains of the sphingolipids and to function as a dynamic glue that keeps the raft assembly together (1).Cholesterol partitions between the raft and the nonraft phase, having higher affinity to raft sphingolipids than to unsaturated phospholipids.Removal of raft cholesterol leads to dissociation of most proteins from rafts and renders them nonfunctional.Association with detergent-resistant membranes (DRMs) is a useful criterion to estimate whether a protein associates with lipid rafts (2).After solubilization of membranes or cells with Triton X-100 or with CHAPS at 4°C, raft-associated lipids and proteins remain insoluble and can then be floated to low density by sucrose gradient centrifugation.If cholesterol is extracted by methyl-β-cyclodextrin or complexed by saponin, the raft proteins usually, but not always, become detergent-soluble.Lipid rafts are first assembled in the Golgi complex in mammalian cells (3).Cholesterol is synthesized in the endoplasmic reticulum (ER), as is ceramide, the hydrophobic backbone of sphingolipids.However, most of the sphingolipid head groups are attached to ceramide in the Golgi complex, where raft assembly takes place (7).There is an increasing concentration of cholesterol and sphingolipids from the ER to the plas-